Recently, reconfigurable intelligent surface (RIS) has been proposed to enhance the coverage and capacity of the wireless communication system with low hardware cost and energy consumption [1]}. In general, RIS consisting of massive passive low-cost elements can be deployed to establish extra links between the base station (BS) and users. By reconfiguring these RIS elements according to the surrounding environment, RIS can provide high beamforming gain [2]}. The reliable beamforming requires accurate channel state information (CSI). Hence, it is essential to develop accurate channel estimation schemes for the RIS assisted wireless communication system [3]}.
